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    ST-120 Issues

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    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    ST-120 Issues

    Post by thermionicvinyl on Sun Nov 11, 2018 6:24 pm

    Hi y’all. I’ve been enjoying my ST-120 for about a year now but it’s been giving me a little bit of trouble this week. A few minutes after turning the amp on a nasty crackling sound starts up in the left channel. It’s very loud and is independent of the input volume. I also noticed the back left tube flashes white joy during this episode.

    Here’s what I’ve tried so far:
    -Replacing the tube in that socket. Result: Problem persists regardless of what tube is in that socket
    -Measuring resistor from pins 1 and 8 to ground on that socket Result: I measure 10.5-11ohm
    -Measuring resistor between pins 5 and 6 on that socket. Result: I measure around 980ohm
    -Cleaning the tube pins with isopropyl alcohol. No change observed.
    -Reheating all the solder joints of pins 1 to 8. Result: This somehow made it worse! The amp still crackles but now there is no sound in both channels

    What have I missed that I can still try? I’m currently using KT120’s bought from Kevin deal along with a WZ68 rectifier.
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    Bob Latino
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    Posts : 2706
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 Issues

    Post by Bob Latino on Sun Nov 11, 2018 7:22 pm

    thermionicvinyl wrote:Hi y’all. I’ve been enjoying my ST-120 for about a year now but it’s been giving me a little bit of trouble this week. A few minutes after turning the amp on a nasty crackling sound starts up in the left channel. It’s very loud and is independent of the input volume. I also noticed the back left tube flashes white joy during this episode.  

    Here’s what I’ve tried so far:
    -Replacing the tube in that socket. Result: Problem persists regardless of what tube is in that socket
    -Measuring resistor from pins 1 and 8 to ground on that socket Result: I measure 10.5-11ohm
    -Measuring resistor between pins 5 and 6 on that socket. Result: I measure around 980ohm
    -Cleaning the tube pins with isopropyl alcohol. No change observed.
    -Reheating all the solder joints of pins 1 to 8. Result: This somehow made it worse! The amp still crackles but now there is no sound in both channels

    What have I missed that I can still try? I’m currently using KT120’s bought from Kevin deal along with a WZ68 rectifier.

    Crackling or static type noises usually indicate a bad driver tube. Buy one NEW 12AU7 type driver tube. Try replacing first the CENTER driver tube. If that does not help, then try using the new tube in the LEFT driver position.

    Bob

    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    Re: ST-120 Issues

    Post by thermionicvinyl on Sun Nov 11, 2018 8:51 pm

    Would that also explain why there is no sound at all?
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    Bob Latino
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    Re: ST-120 Issues

    Post by Bob Latino on Sun Nov 11, 2018 8:57 pm

    thermionicvinyl wrote:Would that also explain why there is no sound at all?

    Yes - it could ..

    The no sound in both channels is usually > A bad center driver tube or a bad rectifier. The way to tell the difference is that a bad rectifier tube also gives a "no bias" situation to all tubes. The most common cause of no sound in either channel is a bad rectifier. The bad center driver tube is less common.

    Bob

    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    Re: ST-120 Issues

    Post by thermionicvinyl on Mon Nov 12, 2018 3:06 pm

    In that case I think I will start by trying a new rectifier then move to replacing one or more of the driver tubes.
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    Peter W.

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    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST-120 Issues

    Post by Peter W. on Mon Nov 12, 2018 3:22 pm

    I strongly suggest you follow Bob's sequence and start with the driver tube(s). A slagged driver tube could damage the rectifier, but a bad rectifier would give you, most likely, not even crackles (at the speakers).

    Best of luck!


    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    Re: ST-120 Issues

    Post by thermionicvinyl on Mon Nov 12, 2018 3:44 pm

    Sounds good. I shall go with some new tubes! I’ll keep this thread updated.

    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    Re: ST-120 Issues

    Post by thermionicvinyl on Wed Nov 14, 2018 11:43 pm

    While I’ve been waiting for some new tubes to arrive I decided to take a close look at the amplifier. First thing I notice is that when I turn it on and wait for the driver tubes to warm up, the centre and the right tube light up but the left tube does not. So I turned off the amp and swapped the centre and the tube to see what would happen. I turned the amp on and surprisingly the left most tube still did not light. It must not be the tube but the amp itself perhaps. I have limited knowledge in electronics so if someone can help me diagnose this it would be much appreciated!
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    pichacker

    Posts : 63
    Join date : 2016-08-01
    Age : 54
    Location : Near to London - UK

    Re: ST-120 Issues

    Post by pichacker on Thu Nov 15, 2018 3:17 am

    Can you post a good closeup photo of the inside of your amp? That way we can see the heater (fillament) wiring connections and possibly spot a dry joint (cold soldered).

    eickmewg

    Posts : 95
    Join date : 2014-08-29

    Re: ST-120 Issues

    Post by eickmewg on Thu Nov 15, 2018 7:04 am

    I had a similar issue with my ST120. Turned out to be a loose connection to one of the heater pins on the tube socket for the left-hand driver.
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    Peter W.

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    Location : Melrose Park, PA

    Re: ST-120 Issues

    Post by Peter W. on Thu Nov 15, 2018 8:29 am

    Where are you? Given your apparent level of skill, I would strongly hesitate to suggest you do anything that involves actual work on the amp. If I knew where you are, I might be able to find a member of some vintage radio club experienced in tube devices and also properly equipped to help. From the symptoms described, it is probably (NOT certainly) quite simple.

    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    Re: ST-120 Issues

    Post by thermionicvinyl on Fri Nov 16, 2018 4:13 pm

    I decided to open up the amp with a friend (more knowledge than me but clueless about this ancient technology) and we immediately spotted a bad a solder joint on the heated pin. Reflowed the solder and now all three drivers light up! Unfortunately the back left output tube continues to arc and crackle. I’ve attached a photo of the tube so perhaps someone can identify the issue: [url=https://servimg.com/view/19998227/1]

    It sounds more like arcing than crackling now

    Edit: I now have sound in the right channel again. Nothing in the left

    thermionicvinyl

    Posts : 18
    Join date : 2018-06-10

    Re: ST-120 Issues

    Post by thermionicvinyl on Sun Nov 18, 2018 6:41 pm

    I’ve monitoring the amp a bit more have been playing different sources through it. Managed to get both channels working but the sound only contains very high frequencies and even at the highest volume level is only very quiet. The back left socket continues to make any tube in it arc and spuder. I’m running out of ideas of what to do with this amp.
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    pichacker

    Posts : 63
    Join date : 2016-08-01
    Age : 54
    Location : Near to London - UK

    Re: ST-120 Issues

    Post by pichacker on Mon Nov 19, 2018 3:54 am

    Can you post a photo as requested earlier please? A disconnected anode connection / OC primay on the OPT will of course mean the screen grid taking the full force...(Speculation)

    If your friend with more knowledge can post here then i'm sure someone on here can help. The amp is fairly simple circuit wise it's just the high voltages that will kill if not treated correctly.

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    Peter W.

    Posts : 1003
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST-120 Issues

    Post by Peter W. on Mon Nov 19, 2018 9:34 am

    thermionicvinyl wrote:I’ve monitoring the amp a bit more have been playing different sources through it. Managed to get both channels working but the sound only contains very high frequencies and even at the highest volume level is only very quiet. The back left socket continues to make any tube in it arc and spuder. I’m running out of ideas of what to do with this amp.

    At this point, I very strongly suggest that you put the amp out of service until it may be checked by a specifically knowledgeable individual with the correct tools and instruments. Everything is 'good' until you toast a transformer, or worse. Equally, I would not even consider remote diagnosis however good the pictures inasmuch as the process from your initial post to the one above is not linear. This is not a criticism by any means or implication. It is simply that what you describe could be:

    a) A bad tube or tubes.
    b) A faulty component or components.
    c) Faulty wiring - although if it ran properly for a year, the fault would be very likely mechanical rather than miswiring.
    d) Something else...
    e) Two-or-more-of-the-above.

    And it is the 'something else' that leads to the suggestion above.

    Consider cascade effects - 'something' causes 'something else', which, in turn causes another 'something'.

    Again, where are you? This amp is not something you wish to ship, casually.

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